1. Field of the Invention
The invention relates to a method of adjusting the inking unit of a printing machine in response to values obtained by scanning or sensing the printing plate, the storage of a previous print order, or by scanning or sensing an original print.
2. Description of Related Arts
The printing process in a printing machine consists basically of conveying printing ink from a reservoir via an inking unit to a printing plate or forme and producing a print from this plate or forme on a printing support. In the case of offset printing, the printing plate also must be dampened and the print transferred from the printing plate to the printing support via a blanket. The printing support is generally paper in the form of sheets or web.
In offset printing there is the specific problem of having to use a printing ink of relatively high viscosity. This is a consequence of the fact that an adequate layer of ink has to be produced on the print support to give an optically adequate print, using very thin ink films. For this purpose, offset printing inks contain very highly concentrated colour pigments and if they are to be usable at all they have to be used in a very viscous state.
The high viscosity of the ink, however, affects the distribution of the ink in the inking unit. The inking unit must be specially constructed to accommodate the high viscosity. A complicated inking unit comprising numerous rollers is usually required to produce a very thin uniform film of ink as required in offset printing. The rollers are usually fed via a vibrator from a duct roller upon which a precise ink profile is set up by ink metering elements. However, the more complex the inking unit construction , the longer it takes before any adjustments made to the ink supply are visible in the print. Experience has shown that continuous printing requires some 300 prints before any adjustment of the ink metering elements reaches the paper and equilibrium is established in the ink transport by the inking unit. The optical impression of the print changes long before this. Dampening of the printing plates is also important in the case of offset printing.
To enable a printing machine to be operated for continuous printing, equilibrium must be established with respect to the ink transport process through the inking unit. Starting with an empty inking unit, for example one cleaned the previous day, a certain quantity of ink is initially required to ensure that all the inking unit rollers are coated with printing ink. This fairly rapidly establishes the ink flow required for continuous printing. In conventional vibrator type inking units, however, it would take a very considerable time to transport to the inking unit rollers the layer of ink required for filling, if it were done solely via the vibrator cycle. Basically, printing ink is needed in the inking unit even where no printing ink is taken from the printing plate during continuous printing.
An additional factor affecting establishing a condition of equilibrium in the inking unit is the "printing plate content" or ratio of printing and non-printing areas on the printing plate and where the printing and non-printing areas are situated. In the printing process, the distribution in the inking unit also builds up a film of ink at places where no printing areas are located on the printing plate.
The objective of the printer is to accelerate the establishment of the state of equilibrium for continuous printing, and typically a manual operation is performed to accelerate the initial distribution of ink particularly traversly of the printing unit. Spreader rollers are provided for this purpose in conventional inking units and are disposed above the first inking unit roller following the vibrator. After filling the duct with ink, the printer applies a strip of ink to the spreader roller, spreading the ink by means of a spatula. The printer will do this particularly where little or no ink is used, because the state of equilibrium is established there only very slowly. The printer then applies the spreader roller manually against the inking unit while the inking unit is running but while it is disconnected from the plate cylinder. The amount of ink applied to the spreader roller is thus distributed throughout the inking unit, where it forms a basic film of ink.
This basic film is, of course, undefined both in terms of thickness and gradient. It is precisely at those places where there is little ink supply that there is already an adequate or possibly even excessive film of ink present. This interferes with the ink feed particularly in the direction of ink transport. The transverse transport by spreading has little effect. The areas having little ink transport in continuous printing are saturated more quickly than would be possible by the normal ink feed.
After the manual spreading operation described above, the ink metering profile set up on the duct roller for the particular printing plate content is introduced to the prepared inking rollers by means of the vibrator. If the printer applied just a sufficient amount of ink during the spreading operation, the incoming ink metering profile is rapidly fed to the inking unit as required. But in practice the results depend basically on the printer's knowledge and experience. It is therefore a question of the printer's feeling for his machine that determines whether optimum results are quickly achieved in continuous printing. Distributing any quantity of ink via the inking unit simply be feel results in an undefinable condition from which the required equilibrium of the ink transport is slowly obtained.
German Offenlegungsschrift No. 2 922 964 corresponding to Canadian Pat. No. 1,137,597 describes a system of printing press preparation and control, in which the inking unit pre-adjustment is described as procedure 5000. This involves using known printing conditions to derive values for adjusting the inking unit of a printing machine. The parameters used are dampening unit settings, machine speed, duct roller rotation, vibrator cycle, plate cylinder and applicator roller diameter and printing plate surface coverage. The thickness of the film of ink required on the applicator roller, and depending thereon the position of the metering elements, are determined from these parameters. Basically, the printing ink itself is included as a parameter. A prerequisite for these calculations is that the inking unit should be in a stable condition, but this means that an adequate quantity of ink must first be present and distributed in the inking unit. The system described, however, can be used only to pre-determine the inking unit setting for the case of continuous printing. In conjunction with the objective of the system, a considerable quantity of spoils are quite deliberately included in the calculations, such spoils occurring during printing until the inking unit is in a state of equilibrium in terms of ink transport.
Control mechanisms have been known for some time in office offset printing machines to automate the sequence of operations of such machines. In these machines the printing plate or foil is automatically fed in, the inking unit and the dampening unit switched on, and the paper transport and printing are initiated. German Offenlegungsschrift No. 2 637 071 corresponding to U.S. Pat. No. 4,084,509 describes a control mechanism for an offset printing machine comprising a pawl and ratchet mechanism by means of which the sequence of operations is automated from the plate feed up to the printing of the first sheet of paper. The paper feed is delayed until the plate has been pre-dampened and has received sufficient ink via the printing unit to produce a saturated print on the blanket. Only then is the first sheet to be printed, and is said to give a good print immediately. However, this system requires a very short inking unit and a relatively low viscosity ink. In addition, the ink profile requirements in such printing machines are very low because they are of course used only for single-colour printing. This means that a uniform film of ink is required over the width of the inking unit, and can be produced easily and rapidly. Also important to the operation of an office offset machine is that there should be no need for further adjustment at the inking unit when the plate is inked and printing starts. The conditions for filling an inking unit of this kind are therefore different from those in offset printing machines which have larger inking units which store the ink. The control mechanism in question therefore uses a fixed transmission system which produces a constant sequence of operations. Since the transmission system is not adjustable, inking of the inking unit is the same for all applications and offers no facility of adapting the ink feed to special cases.